Research into Grinding Hardening of Microalloyed Non-quenched and Tempered Steel

Grinding hardening is a new technology of hardening steel piece surfaces with grinding heat generated in the grinding process instead of with a high or medium frequency induction heating method,which can effectively integrate grinding and surface hardening. Experimental studies were carried out on grinding hardening of non-quenched and tempered steel. Through grinding experiments with variable depths of cut and feeding rate,the variation in the depth of the hardening layer was studied and the microstructure of the hardening zone of the test pieces was subsequently ana-lyzed. In the end,the hardening effect of non-quenched and tempered steel was compared with that of 40Cr steel,which revealed the superiority of non-quenched and tempered steel in grinding hardening technology.

Development of non-quenched and tempered bolt steel

The 8.8 grade non-quenched and tempered bolt steel was studied according to the process conditions of wire rod plant and customer requirments.Three types of experimental steel grades were selected.10MnSiTi Nb and 20Mn2VTi(N) were chosen as the formal steel after several experimemts.

Morphological transformation of elongated MnS inclusions in non-quenched and tempered steel during isothermal heating

Elongated MnS inclusions in rolled non-quenched and tempered steel tend to cause the mechanical anisotropy of steel,deteriorate the mechanical properties and degrade the quality and service life of the steel products.To reveal the mechanisms of morphological transformation of strip-shaped MnS inclusions during isothermal heating,the effects of heat treatment time and temperature on the morphology,number density and size distribution of elongated MnS inclusions were systematically studied and discussed.A diffusion couple experiment was also conducted to clarify the diffusion mode of MnS inclusions.The experimental results showed that with the increase in isothermal heating time(from 0 to 10 h at 1473 K)and temperature(from 1173 to 1573 K for 3.0 h),the number density and average aspect ratio of MnS inclusions generally showed an increase and decrease trend,respectively,while the area fraction remained stable and only slightly fluctuated around 0.4%.In the diffusion couple,after the isothermal heating at 1473 K for 3.0 h,the elements Mn and S in the steel near the steel-MnS interface were very stable without any concentration gradient.The morphology change sequence of the elongated MnS inclusions in the rolled non-quenched and tempered steel during the isothermal heating was strip→cylinderization→spindle→spheroidization.Relationship between the diameter of MnS inclusion and the spacing between two MnS inclusions after splitting,and the fitting goodness of different n values under different experimental time and temperature confirmed that the driving force for the transformation of MnS inclusions during the isothermal heating was surface diffusion,instead of volume diffusion.

Theoretical calculation of the impact work in the alloying non-quenched and tempered steel

Coupled with hot-continuous rolling technology and based on the calculation of the finishing rolling impact work in the non-quenched and tempered Si-Mn steel, the calculations of the finishing rolling impact work in the alloying non-quenched and tempered steel with the elements of Cr, Ni, Mo, W, Cu, V, Nb and Ti are studied with the covalent electron number nA of the strongest bond in alloying phases, the smallest electron density difference ?ρ of phase interfaces, and the number of atom states σ (σ′) which keep the interface electron density continuous. The calculated results show that the finishing rolling impact work of the alloying non-quenched and tempered steel intensely depends on strengthening mechanisms. The solution strengthening, interface strengthening, precipita- tion strengthening of pearlite, and dispersion strengthening will result in the decrease of the finishing rolling impact work; the refinement strengthening, the precipitation strength- ening of V, Nb and Ti in α-Fe-C-V(Nb, Ti), and the residual austenite containing Ni on the boundary of α-Fe-C-Ni will increase the impact work; and the increments or decrements can be calculated with nA, ?ρ, σ (σ′) and weights of alloying elements. The calculation formulas of the finishing rolling impact work in this paper are intergraded with the sug- gested ones of the finishing rolling tensile strength, yield strength, and elongation of the non-quenched and tempered steel. The calculated results agree well with the measured ones.

Theoretical calculation of the finishing rolling impact work in non-quenched and tempered Si-Mn steel

Based on the hot-continuous rolling technology, the finishing rolling impact work α k of the non-quenched and tempered Si-Mn steel is theoretically calculated with the covalent electron number nA of the strongest bond in alloying phases, and the smallest interface electron density difference Δρ of alloying phase interface and the number of atom states σ which keep the interface electron density continuous. Calculations show that the solution strengthening, the precipitation strengthening, and the interface strengthening will result in the decrease of the finishing rolling impact work α k, and the effects of the number of atom states σ which keep the interface electron density continuous on the finishing rolling impact work α k are different. Taking the impact work and the number of atom states σ 0 keeping the electron density continuous of the phase interface α-Fe/α-Fe-C between α-Fe and α-Fe-C as reference values, the impact work of the interface will increase when σ of some interface is larger than σ 0; otherwise, the impact will decrease. Therefore, the finishing rolling impact work α k can be calculated with the impact value of the refined α-Fe matrix and the influence amounts caused by the solution strengthening, the precipitation strengthening, the interface strengthening, and the number of atom states σ which keep the interface electron density continuous. The calculated results agree well with the measured ones. In this paper, the effect of S on the impact work is also discussed.

Hydrogen-assisted fracture features of a high strength ferrite-pearlite steel

Up to now, the exact reason of hydrogen-induced fracture for ferrite-pearlite(FP) steel is still not fully understood. This study presents detail observations of the feature beneath the fracture surface with the aim to reveal the hydrogen-induced cracking initiation and propagation processes. Slow strain rate tensile(SSRT) testing shows that the FP steel is sensitive to hydrogen embrittlement(HE). Focused ion beam(FIB)was used to prepare samples for TEM observations after HE fracture. The corresponding fractographic morphologies of hydrogen charged specimen exhibit intergranular(IG) and quasi-cleavage(QC) fracture feature. Pearlite colony, ferrite/pearlite(F/P) boundary and the adjacent ferrite matrix are found to be responsible for the initial HE fracture and the subsequent propagation. With increasing of the stress intensity factor, fracture mode is found to change from mixed IG and QC to entire QC feature which only occurs at the ferrite matrix. No crack is observed at the ferrite/cementite(F/C) interface. This may be mainly due to the limited pearlite lamella size and relatively low interface energy.

The synergy between cementite spheroidization and Cu alloying on the corrosion resistance of ferrite-pearlite steel in acidic chloride solution

In this work,the corrosion behavior of medium-carbon steels(45,45 Cu and 45 Cuq steels)in acidic chloride environment was investigated.The results indicated that the micro-galvanic effect between the anodic ferrite matrix phase and the cathodic cementite secondary phase notably affected the corrosion resistance of the three steels.For 45 steel,serious pitting corrosion happened in and around the pearlite regions,and a large number of lamellar cementite was fixed in the corrosion pits.Meanwhile,the continuously increasing superficial area of cathodic cementite enhanced the micro-galvanic corrosion,resulting in a rapidly increase in corrosion rate with time.While for 45 Cu and 45 Cuq steels,macroscopic uniform corrosion occurred,and the cementite accumulation was markedly reduced as compared with 45 steel,thus the micro-galvanic effect was weakened and the corrosion rate was decreased accordingly.Among these,45 Cuq steel showed the most stable and excellent corrosion resistance during long-term corrosion,indicating the occurrence of a synergistic effect between cementite spheroidization and Cu alloying,thereby significantly improving the corrosion resistance of 45 steel.

Influence of cementite spheroidization on relieving the micro-galvanic effect of ferrite-pearlite steel in acidic chloride environment

The corrosion behavior of the as-received steel and the spheroidized steel in acidic chloride environment was investigated. The results indicate the corrosion mode and corrosion rate of two steels are diverse due to their difference in microstructure. For as-received steel with ferrite-pearlite microstructure, severe localized corrosion happens on the pearlite regions, and plenty of cathodic cementite remains in the pits, further strengthening the micro-galvanic effect and accelerating the corrosion rate. While for spheroidized steel with tempered martensite microstructure, the nanosized cementite particles evenly distributed on the ferrite substrate are easy to fall off, which can significantly reduce the cementite accumulation on the steel surface, relieving the acceleration effect of micro-galvanic corrosion.

Effect of Nb-V Microalloying on Microstructures and Mechanical Properties of Medium Carbon Non-Quenched and Tempered Steel

Based on optical microscope(OM),transmission electron microscope(TEM) and mechanical performance measurement,the microstructures and mechanical properties of Nb-V micro-alloying non-quenched and tempered steels have been studied.The results showed that the microstructure consists of ferrite and pearlite,in which there exists a lot of intragranular ferrite.Niobium carbide is the main form of carbonitrides,Nb-enriched carbonitrides refine grains,V-enriched carbonitrides have precipitation strengthening effect,which promotes the toughness of the studied steel.The mechanical properties for steels 1,2 and 3 have met the standards required by high load automobile crankshaft,in which the comprehensive property for No.2 is the best.

Review on regulation of MnS in non-qquenched and tempered steel

An overview of the current research status and control methods of MnS in non-quenched and tempered steel was provided.As a low-melting plastic inclusion,the morphology and distribution of MnS were influenced by various production processes.Therefore,control of MnS is a systematic problem that must be integrated into the entire production process.Based on the production process,the factors affecting the morphology and distribution of MnS in steel were introduced.The effects of oxygen activity,manganese,sulfur,and some alloys on MnS inclusion precipitation were summarized,mainly including MnS modification treatment and oxygen-sulfide composite precipitation control.It is believed that MnS precipitates during the solidification process of steel,and controlling the solidification cooling rate could effectively regulate the size and morphology of MnS,avoiding the precipitation of II-MnS.Additionally,by changing the deformation rate,deformation amount,deformation temperature during the hot deformation process,and heating time and temperature during heat treatment,the distribution and morphology of MnS could be improved.Through the fine control of the above process parameters,the number of II-MnS in steel could be effectively reduced,and their morphology could be improved,thereby enhancing the performance of non-quenched and tempered steel and promoting its wider application.Furthermore,applying laboratory research results to industrial production is an important direction for future research efforts in this field.

Prediction of mechanical behavior of ferrite-pearlite steel

A new approach describing the flow stress of ferrite-pearlite steel has been proposed,which divided the deformation process into three stages based on whether ferrite or pearlite yielded. Iso-work increment assumption was applied to describe the transfer of load between the components. The physically based model to describe ferrite was approximated with Swift＇s equation in order to obtain the analytic solution.The tensile strength of ferrite-pearlite had a linear relation with pearlite volume fraction,square root reciprocal of ferrite grain size and reciprocal of pearlite interlamellar spacing. Moreover,a model to calculate the tensile strength of ferrite-pearlite steel was proposed. The predicted values of tensile strength were in good agreement with experimental results when the pearlite volume fraction was less than 20%.Considering the plastic relaxation mechanisms,the internal stress was modified with pearlite volume fraction,total strain,yield stress of ferrite and pearlite when the pearlite volume fraction was more than 20%.

Microstructure and strength in ultrastrong cold-drawn medium carbon steel

Via traditional wire drawing,the medium carbon ferrite-pearlite(MCFP)steel wires can achieve the ultrahigh strength beyond 4 GPa normally for high-carbon pearlitic steel wires,but have a 30-60%lower production cost.The microstructural evolution and mechanical properties of medium carbon ferrite-pearlite steel wires have been investigated by means of scanning electron microscopy,transmission electron microscopy and tensile testing.The tensile strength of medium carbon ferrite-pearlite steel wires increases from 750 MPa up to 4120 MPa when the drawing strain increases up toε=6.4,which represents the highest strength reported so far-to our knowledge for a carbon steel with such low carbon content.At low and medium strains(ε≤1.95),the proeutectoid ferrite forms dense dislocation walls(DDWs)via dislocation activities,including sliding,accumulation,interaction,and tangling.With the drawing strain increase,the reorientation of DDWs to the drawing direction forms the coarse proeutectoid ferrite lamellae.Finally,the proeutectoid ferrite deformed to high strains is characterized by a lamellar morphology and the average lamellar spacing of proeutectoid ferrite is about 55 nm atε=6.4.The interlamellar spacing of pearlite and thickness of cementite decreases with the drawing strain increases.The dislocation density in ferrite lamellae increases with the drawing strain increases,and the dislocation density in ferrite lamellae is 7.8×10^(15)m^(2)atε=4.19.A higher dislocation density of 3.1×10^(16)m^(2)can be obtained atε=6.4 by means of extrapolation and TEM investigations.The stress contributions of proeutectoid ferrite and pearlite to the flow stress are estimated based on quantified structural parameters.Based on the assumption that the stress contributions from different strengthening mechanisms are linearly additive and the general rule of mixtures,a good agreement between the measured and estimated flow stresses has been found in a large range of flow stress.The good application of the general rule of mixture to the medium carbon ferrite-pearlite steel wires indicates the importance of quantitative characterization of microstructural evolution and parameters with the strain.

Effects of metallic microstructures on fatigue fracture of Q345 steel

Effects of high-frequency cyclic loading on the banded ferrite-pearlite steel were analyzed through crack initiation and propagation. Interfaces of ferrite and pearlite colony with a small angle deviation from the loading axis were verified to be the most potential sites to fabricate the microcracks caused by the high strain gradient. The initial crack extension inside ferrite grain was driven by shear stress in model II along the direction with a 45° angle to the loading axis. Banded pearlite colony and the high-angle grain boundaries were considered as the dominant factors that promote the fatigue resistance of the material through arousing crack deflection in short crack propagation range and crack branching in long crack propagation range to reduce the crack propagation driving force in the crack tip. P-S-N curves were used to quantify the dispersion of fatigue lifetimes and evaluate the effect of elevated volume content of pearlite colony on the fatigue performance of the material.